1. Field of the Invention
This invention relates to a viewer for diffusion plates which provides magnified dark-field viewing of the plates and includes means for optically measuring the diameter of rings on the plate.
2. Prior Art
Immuno-diffusion plates are commonly used to quantitate constituents of blood and other body fluids. The unknown specimen is applied to a narrowly defined area of a plate coated with a gel and reacts with the unknown to form a precipitate. As the unknown diffuses radially from its point of deposit a ring of precipitate is formed by the reaction of the unknown with the gel and the dimensions of this ring can be related to the quantity of the unknown in the sample, typically by comparison with the dimensions of rings produced by samples containing known quantities of the constituent in question.
In the past the dimensions of these rings have typically been measured employing plastic scales or grids which are positioned directly above the ring. These scales cannot be placed in contact with the plate or they will damage the fragile gel. Their use accordingly tests the agility and visual acuity of the observer and the readings obtained are limited in precision because of the parallax resulting from the necessary spacing between the scale and the plate and the ability of the observer to make an accurate reading without a magnified scale.
Other prior art systems use a magnifier and a graduated reticle. Like the scale, the reticle is supported in close proximity to the plate but in a different plane, producing both focusing and parallax problems.
These prior art devices also relied on the observer's ability to distinguish the bounds of the diffusion ring which is substantially the same color as the surrounding gel and may only be distinguished on the basis of its slightly higher optical diffusing properties. These problems render the prior art devices difficult to use, of limited accuracy and highly susceptible to erroneous readings.